HIV and the lentiviruses of animals cause persistant, minimally productive infections that lead to degenerative changes in multiple organ systems and invariably the CNS. The purpose of this proposal is to determine mechanisms of neuroinvasiveness and neurovirulence of visna virus of sheep, a lentivirus which shares many molecular, biological and pathogenetic characteristics with HIV. The central hypothesis is that neurovirulence is related to an increase in virus gene expression in brain macrophages and that this is followed by a burst of cytokine production causing a cascade of immunological effects. We have produced a neurovirulent strain of visna virus (strain NV93) by passing a field strain of visna (P93) multiple times in the brains of sheep. NV93 causes rapid onset of severe, characteristics neurological lesions of visna when inoculated intracerebrally into sheep. Using sheep inoculated with NV93 or P93 and killed sequentially, we will characterize the pathological changes in the CNS and identify cells in the neuropil that support viral replication using immunocytochemistry for cell makers and in situ hybridization to detect viral transcripts. The onset of lesions in sheep inoculated with NV93 may correlate with viral gene expression. Using sheep inoculated by the intravenous route and killed sequentially, the sequence of infection in non-neural tissues will identified and the portal of entry to the brain will be identified. Replication on NV93 and P93 will be studied in culture primary neural target cells. Particular attention will be paid to viral gene expression in brain macrophages and vascular endothelial cells. Cultures of primary neural cells will be used in studies to determine whether neurovirulence correlates with excessive production of , or response to cytokines produced by brain cells.